The present application is related to European Patent Office Patent Applications EP-01300493.2 entitled xe2x80x9cStair Step Current CPT Measurement Method and Apparatusxe2x80x9d filed on Jan. 19, 2001 in the European Patent Office and European Patent Office Patent Application EP-01305831.0 entitled xe2x80x9cStair Step Voltage Actuated Measurement Method and Apparatusxe2x80x9d filed on Jun. 22, 2001 in the European Patent Office. Both of these patent applications are in the name of inventor James Lee Hedgecock and are commonly owned herewith.
The present invention relates to the field of medical science, and particularly although not exclusively to a method of and apparatus for utilizing bio-electric stimulation.
It is known to perform experimental examination for identifying abnormalities in nerve fibers, by applying an electrical stimulation transcutaneously to a patient.
It is well documented that specific current signal frequencies will selectively stimulate distinct types of nerve fibers, for example 5 Hz selectively stimulates type C nerve fibers, 250 Hz stimulates type A-Delta nerve fibers, and 2000 Hz stimulates type A-Beta nerve fibers. This neuroselectivity of frequencies is exploited by a method employed to measure the lowest level of current intensity a subject can recognize with a transcutaneous electrical stimulus. This method is termed current perception threshold (CPT) diagnosis.
A major problem often encountered during CPT testing is that the cutaneous electrical resistance threshold (CERT), the current signal level required before conduction through skin can occur, may be greater than the current perception threshold, the current at which a patient recognizes that a nerve has been stimulated. In subjects with a greater CERT than CPT the intensity of the diagnosis signal is turned up past the actual CPT without the subject recognizing the stimulus, since the current is not flowing through the skin to the nerve fiber. Once the intensity reaches the CERT and the current begins flowing, then the subject may report a false xe2x80x9chighxe2x80x9d CPT, which is actually the CERT being reached by the applied signal.
Previously, a xe2x80x9cconstant currentxe2x80x9d mechanism of bio-electric stimulation was developed in the 1950s and refined in the early 1980s. This later refinement is disclosed in U.S. Pat. No. 4,305,402 (Katim). Katim""s constant current mechanism monitored a sine wave current and regulated it so that once the CERT had been reached the current was maintained automatically so as to sustain the signal intensity at sufficient level to allow a continuous flow of current, even though a manually operated intensity control may be turned to zero. Thereby, on the next measurement in a serial test at a same skin site on a patient, the current is not required again to breach the CERT, and the actual CPT can thereby be more accurately measured.
Katim""s constant current mechanism works best with a sinusoidal wave form current. However, a sinusoidal current is quite difficult for a patient to recognize within a very narrow range of intensities. Due to the wide fluctuation in measurements obtained using a sinusoidal voltage, measurements must be averaged before meaningful analysis is possible.
A more recognizable stimulus is that of a modulated square wave signal, and in particular, a modulated square wave current. A square wave form current is used in the prior art Medi-DX 7000 CPT diagnostic device of Neuro-DX Associates Incorporated, 445 Dartmoor Street, Laguna Beach, Calif., 92651-1430. This device enables location and quantification of nerve pathology caused by injury, metabolic, and toxic exposures, and provides a screening method for patients prior to invasive examinations and procedures are undertaken. Results of up to 95% accuracy in the detection and quantification of nerve pathology are achievable.
The U.S. Pat. No. 6,029,090 (Herbst) discloses a multi-functional electrical stimulation system having a variety of wave forms including a sine, saw-toothed or square wave form. As with similar prior art stimulation devices Herbst""s device provides for a wave form that may be customized in terms of pulse widths and pulse repetition rates. Similarly, GB 2123698 (Biostim) discloses a biological electrical stimulator capable of generating a variety of electrical stimulation wave forms being adjustable with regard to amplitude, pulse rate and burst.
Further teachings of the use of a square wave form being an electrical stimulation signal can be found in U.S. Pat. No. 5,797,854 (Hedgecock), U.S. Pat. No. 4,646,744 (Zion), U.S. Pat. No. 4,690,145 (Minnesota Mining) and U.S. Pat. No. 5,020,542 (Roosmann) however, none of the aforementioned references address the issue of maintaining a flow of current through the skin to the nerve fiber when attempting to measure and determine the CPT in subjects with a greater CERT.
Referring to FIG. 1 herein, there is illustrated schematically in perspective view, the known Medi-DX 7000 current perception threshold diagnostic device. The device comprises a casing 100 containing drive electronics for performing current perception threshold measurements on a patient, the casing having a front panel 101 having a first electrical connector port 102 for connection of a probe device 103; a second electrical connector port 104 for a defuse area electrical contact 105; a set of frequency selector switches 106-108 respectively, for selecting test signals having fundamental frequencies corresponding to 5 Hz, 250 Hz and 2 kHz, for testing type C nerves, type A delta nerves, and A-beta nerves respectively; a current intensity control 109 in the form of a rotary dial, having a graduated scale around a circumference of the dial, the rotary dial capable of varying an output current signal in the range 0 to 10 mA between the probe 103 and second electrical contact 105; a liquid crystal display device 110 used to calibrate the current amplitude during manufacture and during after sales service; and an on/off power switch 111.
The usage of the device is known in the art, and is as follows:
A patient is placed into a relaxed position by a medical personnel. The second electrode contact 105 is placed upon a region of the patient""s skin to make electrical contact. The second contact 105 is immersed in saline solution, to improve conductivity between the skin and a wide area contact region of the second contact 105. The probe 103, comprises a gold plated tubular contact 200 capable of receiving a cotton bud 203, which is dipped in saline solution to improve conductivity between the gold contact 200 and a patient""s skin as illustrated in FIG. 2 herein. The probe 103 and second electrical contact 105 are placed at various positions around the patient""s body, and a square wave electrical signal is passed between the probes through the patient""s skin, in order to test various nerves around the patient""s body as is known in the art.
To test a particular nerve, the medical operator places the probe and contact at specified positions on the persons body and starting from a zero reading, on the rotary current dial 109 corresponding to zero mean current and gradually increases manually the current by rotating the current control 109, until the patient indicates that a sensation is felt. Due to variations in connection resistance between the cotton bud on the end of the probe 103, and the patient""s skin, the medical operator repeats this process 3 or more times for every measurement position, in order to reject spurious readings, and to take a set of readings which are consistent with each other, and which can be used to derive an average reading. Since the medical operator relies upon the patient""s perception of sensation due to current, the patient may, either voluntarily or involuntarily, give a misleading indication of when a sensation is felt. For example a patient may, by the intonation of the human medical operator""s voice, anticipate when to indicate sensation. Therefore the operator must be careful not to give any indication to the patient of when a sensation could be expected.
For each nerve tested, the operator manually fills in a record sheet similarly as illustrated in FIG. 3 herein. For example for a cervical test, nerves from the C2 to Thoracic 2 nerve may be tested, on both the left side of the patient""s body and the right side of the patient""s body and entered onto the record sheet as a current intensity reading on a scale 0-100, corresponding to a peak current of 0 to 10 mA. Similarly the medical operator places the probe 103 and second contact 105 on the patient""s skin and records readings for the Lumbar L1 to S2 nerves, and other nerve groups as is known in the art.
However, there is a problem in measurement, arising from the electrical conductivity characteristics of a patient""s skin. When the probe 103 and second contact 105 are applied to a patient""s skin, electrical contact is made via the saline solution, for a current value which has a peak to peak value of a skin conductivity threshold value (CERT), which is determined by the conductive characteristics of the patient""s skin. Once the CERT is breached and current is flowing through the skin, provided electrical contact is not lost, and provided the current values does not fall too far below the cutaneous electrical resistance threshold, then current will continue to flow even below the CERT.
However, there is a difficulty in recording readings if the operator returns the rotary current control dial 109 to give a signal too far below the skin""s CERT value. Once the current either falls too far below the skin conduction threshold value and current ceases to flow, or if the electrical contact is broken, at a current value below the CERT then the operator must again increase current to exceed the threshold value, before readings can recommence. Additionally, for nerves which respond to currents near the skin conduction threshold value, obtaining accurate readings is made more difficult.
On some patients, the CERT for some skin sites is higher than the current perception level (CPT). In these cases, measurement is difficult because the operator must first breach the CERT, and reduce the current below the CERT, whilst still maintaining current conduction through the skin, in order to test the patients current perception threshold. If at any time the operator reduces the current dial to zero, current conduction will be lost, similarly if the electrical contact between the probe and the skin is broken, then current ceases and measurements must be re-started at the same site. The current value at which current flow stops is characteristic of each individual patient, and is not a known fixed number.
The problem is exemplified, by the plot of peak to peak current versus time shown in FIG. 4 herein which plots an example of the peak to peak current, as controlled by the medical operator, relative to a skin conduction threshold level 401 and a nerve sensitivity threshold level 402.
In this example, the CERT is above the CPT. Initially, the operator turns the current dial from zero up to, for example a reading of 45, at which point the patient indicates that sensation can be felt. This could either be the CERT, or the CPT. At this stage the operator cannot tell which. The operator therefore reduces the current down to a lower reading of 10, and raises the current again slowly. When the dial reads 30, (the level of the CPT) the patient indicates that a sensation is felt, therefore this is likely to be the CPT. However, to verify that, the operator again drops the current to a value of 10, and slowly raises it through the 30 level, at which the patient again indicates a sensation. To verify this a further time, the operator reduces the current back to 10 and raises slowly through 30 at which point the patient verifies a sensation at the current level of 30. In this case, the CERT has a value of 45, and the measured CPT has a value of 30.
The user protocol to deal with this measurement includes:
At no time can the probe or electrical contact be lifted from the skin sites.
At no time during the sequence of readings can the intensity be turned to zero (and ideally should not be reduced below a reading of 10 corresponding to 1.0 mA).
Once the current is turned down, the patient is asked if they continue to feel a stimulus, and if not, then the current is turned up until they feel the stimulus again.
If an initial higher current measurement is found, then the operator suspects that it is possibly the CERT reading. The operator then turns down the current, but not so far as previously, and asks if the patient continues to feel a stimulus. The operator then turns up the current again until the patient indicates stimulus is felt.
If the same high reading is noted as previously, then that is the actual CPT and it is a true high reading. Otherwise the initial high reading is a breach of the CERT, and the CPT lies below the initial high reading.
As another example, an operator may initially increase the current to a reading of 45 (4.5 mA) turn down the current to 10 and raise it again for a second reading at 45. The current is then turned down to 20 and a third stimulus reading is recorded at a value of 38. Subsequently the current is turned down to 20 again and a fourth reading is measured at a current of 38 and similarly the fourth or fifth reading also a current is measured at 38. In this case, the CPT is at a value of 38 (3.8 mA) and the CERT (the first two readings) is at a value of 45 (4.5 mA).
If the current is allowed to drop below a critical level at which the current stops (typically between 0 and 10) then current conduction through the skin ceases and the measurement sequence must be started again.
Therefore, to obtain a reliable set of readings, the medical operator must not vary the probe contact to the skin between readings, and ideally should not let the current drop to zero during a set of consecutive readings.
The inventor has recognized various problems associated with the utilization of bio-electric stimulation for medical diagnosis based on the variation and monitoring of a current. As the aim of the medical diagnosis applied to a patient is to assess the operation of a nerve and in particular a nerve impulse, and given a nerve impulse is caused by, and is directly correlated with voltage the above diagnosis based on current perception threshold provides an indirect measurement of a nerve impulse. A direct measure of a nerve impulse being provided by the monitoring of voltage intensity.
The inventor has further realized that a more recognizable stimulus signal than a sine wave or square wave is that of a modulated square wave form as shown in FIG. 5 herein, in which a duty cycle is controlled so as to produce an interval of zero voltage between an alternating positive and negative square wave cycle. However, it has been noticed that this brief interval of zero voltage can interrupt the current flow, especially when testing with lower frequencies. This interruption necessitates breaching the CERT again, with a false high sensory threshold measurement.
The bio-electric stimulation method as disclosed in Katim and various of the highlighted prior art, as used to provide an assessment of nerve impulses, assumes, according to Ohm""s Law, that the patient""s skin has a constant and unchanging electrical resistance. However, this premise is false as, in reality, the skin""s resistance is constantly changing and moreover is increasingly varied when current is applied to its surface as in the CPT method, whereby the skin is over-stimulated to release histamine and like substances such that its electrical resistance is substantially altered.
As the method of diagnosis, disclosed herein, involves the measurement of voltage intensity a new terminology is adopted to mirror that of the previously discussed current perception threshold (CPT), this new terminology being Voltage-Actuated Sensory Nerve Conduction Threshold (V-sNCT).
The inventor, through the specific implementations of the present invention, aims to address the above identified problems, namely the inherent inaccuracies involved with a bio-electric stimulation and diagnosis based on current signals and in particular the associated use of bio-electric stimulation using a wave form in the form of a sign or square wave. Based on the fact that the change in a membrane potential, which initiates the nerve impulse, is caused by, and directly correlates with voltage intensity; specific implementations of the present invention aim to provide a method and apparatus utilizing bio-electric stimulation for medical diagnosis based on a direct measurement of the nerve impulse by the measurement of change in membrane potentialxe2x80x94measured in volts. The single similarity between the previously discussed CPT diagnosis and the V-sNCT diagnosis according to the specific implementations of the present invention, is that both methods use Neuro-selective frequencies to access the type C fibers (5 Hz), A-Delta (250 Hz) and a A-Beta fibers (2 kHz).
Specific implementations of the present invention further aim to correct the problem of interruption of stimulus signal electric flow by use of a unique electrical wave form which has not been employed previously in experimental bio-electrical stimulation, or in any device used for nerve impulse testing. This new wave form is similar to a modulated square wave form signal with a zero interval, however instead of an interval between square waves being zero, the voltage steps down a controllable percentage of a preceding major part of the wave form. A circuit controls this step voltage as a percentage of a major stimulus voltage. Therefore current flow is maintained since current does not completely stop flowing through the patient""s skin. The step voltage is maintained at its highest level until a stair step reset operation returns the step voltage to zero before a new site (nerve) is tested. This mechanism allows any major stimulation wave form to be decreased, but not below a predetermined lower step voltage value.
One object according to the specific implementations of the present invention is to provide a single wave form which maintains current conduction through skin during variation of a stimulus voltage signal amplitude.
A second object of specific implementations of the present invention is to improve the usability of a measurement apparatus using bio-electric stimulation.
According to a first aspect of the present invention there is provided a method of measuring sensitivity of a patient to electrical stimulation comprising the steps of:
attaching a source of electrical signal to a skin region of said patient;
applying an alternating electrical signal from said source, said signal comprising a stimulation signal element for providing electrical stimulation to said patient, and a step signal element for maintaining a flow of said electrical signal through said skin region, said step signal element having an amplitude being set as a percentage of an amplitude of said stimulation signal element; and
recording a value of said stimulation signal element at which a nerve stimulation is identified.
Preferably, wherein said electrical stimulation signal is characterized by a wave form having:
a first leading edge raising to a maximum amplitude;
a first trailing edge dropping from said maximum amplitude to a step amplitude, said step amplitude applied for a time duration after said first trailing edge; and
a second trailing edge transition reducing said amplitude from said step level, to a zero amplitude.
According to a second aspect of the present invention there is provided an apparatus for applying an electric stimulation signal to a patient, said apparatus comprising:
a signal generator circuit for generating an electrical stimulation signal;
a display device for generating a display of a value of said stimulation signal;
a frequency selector circuit, for selecting a frequency of said electrical stimulation signal; and
first and second electrical contacts for making contact of said electrical stimulation signal with a skin region of a patient, characterized in that:
said electrical stimulation signal is alternating, having a positive cycle and a negative cycle, wherein said stimulation signal has a step signal element for maintaining a flow of said electrical signal through said skin region of said patient, said step signal element having an amplitude being set as a percentage of an amplitude of said electrical stimulation signal.
According to a third aspect of the present invention there is provided a method of generating an electrical stimulation voltage for voltage actuated sensory nerve conduction threshold measurements, said method comprising the steps of:
setting a first amplitude level, of a first portion of a cycle of said voltage;
setting a second amplitude level over a second portion of said cycle;
setting said second amplitude level dependent upon said first amplitude level; and
setting said second amplitude portion to be maintained at or above a predetermined minimum amplitude value.